A rupture disk is disposed between two standard pipe flanges along a vent pipe or relief port of a pressurized vessel. One side of the rupture disk faces the pressure vessel (referred to as the process side of the rupture disk) and is subject to the pressure in the pressure vessel. The opposite side of the rupture disk faces a piping system coupled to the vent pipe (referred to as the vent side of the rupture disk) and is subject to the pressure in the piping system. If the pressure in the vessel increases and the pressure differential between the process side and the vent side of the rupture disk exceeds a predetermined burst pressure rating of the rupture disk, the disk fragments or ruptures allowing release of pressure in the vessel though the vent pipe thereby avoiding failure of the pressure vessel.
Because of environmental concerns and regulatory requirements, fugitive gases exiting from the pressure vessel through the vent pipe upon rupture of the rupture disk are typically not permitted to be vented directly to the environment, for example, substances in the vessel may be toxic, carcinogenic, radioactive or otherwise harmful to the environment. Instead, the pressure vessel vent pipe is coupled to a piping system is utilized to collect fugitive discharge, that is, material, typically gas, discharged as a result of rupture of a rupture disk. In a facility with multiple pressure vessels, the piping system may be one of a number of types including common header and manifold. In a manifold piping system, individual vent pipes extend from each of the pressure vessels to a catch tank or discharge collection vessel. In a common header piping system, a single pipeline or conduit extends to a catch tank. The vent pipes from a plurality of pressure vessels are routed into the common header pipeline at various points along the pipeline.
If the maximum allowable pressures in a plurality of pressure vessels differ between vessels, then rupture disks having different burst ratings are required. For example, if the maximum allowable pressure in a first pressure vessel is 10 pounds per square inch gauge pressure (psig) and the maximum allowable pressure in a second pressure vessel is 100 psig, then first and second rupture disks having burst pressure ratings of 10 psig and 100 psig are installed between flanges in the vent pipes of the first and second pressure vessels respectively.
In a common header piping system, utilizing a plurality of rupture disks having different burst pressure ratings presents a problem because of short duration vent side or back pressure surges that result from fugitive discharges. That is, a low burst pressure rupture disk may rupture as a result of excess vent side pressure in the common header pipeline from a fugitive discharge even though the pressure in the pressure vessel is below the maximum burst pressure rating of the rupture disk. For example, if the pressure in the second pressure vessel increases and the pressure differential between the process and vent sides of the second rupture disk exceeds 100 psig, the second rupture disk will burst and fugitive discharge gas from the second vessel will be discharged through its vent pipe into the common header pipeline and be routed to the catch tank. Upon discharge of 100 psig fugitive discharge gas from the second pressure vessel, the pressure in the common header pipeline will increase as the fugitive gas enters the pipeline.
The pressure applied to the vent side of the first rupture disk as a result of fugitive discharge gas entering the common header pipeline from the second pressure vessel depends on various factors including the volume of discharge gas escaping from the second pressure vessel, the internal diameter of the common header pipeline and the distance along the common header pipeline between the first and second pressure vessel vent pipes. Under certain conditions, the vent side back pressure in the common header pipeline adjacent the first pressure vessel vent pipe may be a sufficient magnitude to create a pressure differential on the first rupture disk that causes the first rupture disk to fail or rupture due to vent side back pressure. It is important to note that a magnitude of pressure that results in failure of a rupture disk due to vent side back pressure is usually not the same as the process side burst pressure rating. While it is crucial that a rupture disk rupture when the process side burst pressure is exceeded (that is, the rupture disk ruptures in a direction of flow from process to vent), it is very undesirable for the rupture disk to fail in the opposite direction (that is, in a direction of flow from vent to process) due to excess vent side back pressure. An undesired rupture of the first rupture disk due to excess vent side back pressure results in contamination of the materials in the first pressure vessel by the fugitive gas in the common header pipeline as well as downtime, materials and maintenance costs associated with replacing the first rupture disk, cleaning the contaminated materials in the first pressure vessel, etc.
Any piping design where there is venting from vessels to a common piping system is subject to the risk of vent side back pressure undesirably rupturing or causing failure of a rupture disk, not just a common header piping system. However, the problem is more pronounced in a piping systems where the length of pipe separating the various pressure vessels is relatively short. A common header piping system typically will have a shorter piping distance between adjacent pressure vessels than, for example, a manifold piping system where each pressure vessel has its own individual vent pipe extending to the catch tank.
One solution to the problem of undesired vent side or back pressure ruptures is a line of rupture disks sold by Zook Enterprises, LLC of Chagrin Falls, Ohio, the assignee of the present invention, under the trade name Zook.RTM. Bak-Pressure.TM. disk. Bak-Pressure.TM. disk are offered in burst ratings from 1/4 psig to over 1000 psig. Each Bak-Pressure.TM. Disk product is a rupture disk assembly that includes a graphite rupture disk and additionally may include a graphite or metal support member.
The graphite rupture disk comprises an annular support portion surrounding a thin pressure sensitive rupture membrane adjacent a first end of the annular support portion. The membrane blocks the flow of fluid in the vent pipe and is integral with the annular support portion. The rupture membrane is flush with the first end of the annular support portion and the membrane together define a substantially planar outer surface or wall of the rupture disk. The rupture membrane will break or rupture if the process side pressure exceeds the vent side or back pressure by more than a predetermined pressure rating of the rupture disk. A second end of the annular support portion and rupture membrane define a central cylindrical shaped recess extending inwardly from a second end of the support portion to the rupture membrane.
A graphite rupture disk may be mounted in two orientations: a mono orientation and an inverted orientation. In the mono orientation, the second end of the annular support portion of the rupture disk, that is, the end having the central cylindrical recess, faces the process side, that is, the pressure vessel. In the inverted orientation, the first end of the annular support portion, that is, the planar side of the rupture disk, faces the process side.
An important characteristic of a graphite rupture disk is that the burst pressure of the rupture disk is dependent upon the orientation of the rupture disk. Thus, when specifying the burst pressure of a graphite rupture disk the orientation of the rupture disk must also be specified. The relationship between the burst pressures of a rupture disk when mounted in the mono orientation versus the inverted orientation is dependent upon the diameter of the rupture disk and the thickness of the rupture disk membrane, but a general rule is that the burst pressure of the rupture disk in the mono orientation will be less than or equal to the burst pressure of the same rupture disk in the inverted orientation.
For example, a rupture disk having a very thin rupture membrane will have a burst pressure that is substantially equal in both the mono and inverted orientations, e.g., a 1 inch diameter rupture disk having a process side burst pressure of 10 psig in the mono orientation would also have a process side burst pressure of approximately 10 psig in the inverted orientation. As the thickness of the rupture membrane increases the difference between the burst pressure of a rupture disk depending on orientation will increase, e.g., an 4 inch diameter rupture disk having a process side burst pressure of 100 psig in the mono orientation would have a process side burst pressure of approximately 125 psig in the inverted orientation. A 4 inch diameter rupture disk having a process side burst pressure of 500 psig in the mono orientation would have a process side burst pressure of approximately 1000 psig in the inverted orientation.
A Zook.RTM. Bak-Pressure.TM. disk is mounted between flanges of a pressure vessel vent pipe in the mono orientation. Since the rupture disk is mounted in the mono orientation, the vent side back pressure that will cause failure or rupture of the disk will be at least equal to an typically greater than the process side burst pressure rating Under certain conditions, sufficient vent side or back pressure protection is provided by the vent side back pressure capability of the product's rupture disk. If a greater vent side pressure protection magnitude is desired, the Bak-Pressure.TM. disk may include a graphite or metal support. Such a support reinforces the rupture membrane and increases the product's capability to withstand vent side back pressure without failure.
Although the Zook.RTM. Bak-Pressure.TM. disks have been a commercial success, one shortcoming of such disks is that they do not permit installation of the rupture disk in the inverted orientation, that is, where the flat or planar side of the rupture disk faces the process side. The inverted orientation is advantageous because it is easier to affix protective corrosion and/or temperature resistant overlays (a layer and/or a coating) to a flat surface than to the cylindrical shaped recessed surface facing the process side when the disk is positioned in the mono orientation.
Thus, it would be desirable to have a rupture disk assembly that is adapted to be used in the inverted orientation in a pressure vessel vent pipe while still providing protection against undesired vent side back pressure rupture of the rupture disk.